In recent years, the light emission efficiency of a semiconductor light source such as an LED (light-emitting diode) and an organic EL element has been improved, and illumination devices employing the semiconductor light sources have been put into practical use. The improvement in the light emission efficiency of the semiconductor light source has been recognized in the field of motor vehicles. Commercially available are motor vehicles that make use of semiconductor light sources in headlamps, taillights or brakelight.
In general, a lighting device for use in turning on a semiconductor light source is designed to turn on the semiconductor light sources through constant current control by which an electric current flowing to the semiconductor light sources is kept constant. This is to make uniform the light flux of the semiconductor light sources independent of a power supply voltage or a forward voltage of the semiconductor light sources.
There are proposed lighting devices, each having a load changeover function (a multiple-load control function) for changing over target loads (e.g., semiconductor light sources) to be powered such as taillights and a brakelights or crossing headlights and curvelights employed in motor vehicles. As one example, there is known a lighting device of a configuration in which semiconductor light sources are connected in series to an output terminal thereof. In this lighting device, a light output is reduced by short-circuiting some of the semiconductor light sources and keeping them turned off (see, e.g., Japanese Patent Application Publication No. 2004-39288, Paragraph 0045).
FIG. 18 shows one exemplary lighting device 1′ having such a load changeover function. In the lighting device 1′, loads 2 including a plurality of (nine, in FIG. 18) LEDs 21 and 22 serially-connected are turned on by applying a DC voltage thereto. The lighting device 1′ includes a bypass switch (MOSFET) 4 connected in parallel to some of the LEDs (three LEDs 22 at the cathode side). The lighting device 1′ can turn on the LEDs 21 and 22 with the bypass switch 4 switched off, and then turn on only the bypass switch such that only LEDs 21 are being kept on while the LEDs 22 are being short-circuited.
The lighting device 1′ shown in FIG. 18 includes a current measuring circuit 7 for measuring an electric current flowing through the loads 2 as an output current. The lighting device 1′ realizes constant current control by driving a power converting unit (DC-DC converter) 3 in such a manner that the average value of the output currents can be kept equal to a predetermined designated current value.
The lighting device 1′ shown in FIG. 18 further includes an abnormality detecting unit 56 that stops the operation of the power converting unit 3 by detecting an abnormality from a measurement result of a current measuring circuit 7 or a voltage measuring circuit 6, which measures a voltage applied to the load 2 as an output voltage. The abnormality detecting unit 56 stops the operation of the power converting unit 3 by determining an abnormality when the output voltage becomes falling out of a specific normal range (e.g., from 10V to 40V).
(a) and (b) of FIG. 19 respectively show the changes in an output voltage and an output current in the lighting device 1′ of the configuration described above. In FIGS. 19 through 23, the horizontal axis indicates the lapsed time. The vertical axis in (a) of each of FIGS. 19 to 23 indicates an output voltage while the vertical axis in (b) of each of FIGS. 19 to 23 indicates an output current.
When the LEDs 21 need to be turn on, the lighting device 1′ drives the power converting unit 3 to increase an output voltage. An output current begins to flow when the output voltage reaches a forward voltage Vf1 of the LEDs 21. In this regard, the output current remains constant (the output current is assumed to be 0.7 A herein, but is not limited thereto) because the lighting device 1′ performs the constant current control.
Further, if the LEDs 22 need to be turn on additionally in this state, the lighting device 1′ performing the constant current control increases the output voltage in proportion to the added forward voltage Vf2 of the LEDs 22 so that the output current can be kept constant. This makes it possible to realize the constant current control with respect to the loads 2 and to change over the loads 2 to be powered while turning on the LEDs 21 and 22 with constant light flux regardless of the power supply voltage.
Focusing on the short period of time immediately after the loads 2 are changed over, the output voltage is overshot with the output current reduced sharply as shown in (a) and (b) of FIG. 20. These phenomena are caused by the time delay attributable to the fact that the output control of the lighting device 1′ is a feedback control performed after detection of the output current. It is difficult to prevent occurrence of such phenomena.
Inasmuch as variations exist in the forward voltages Vf (=Vf1+Vf2) of the LEDs 21 and 22, the output voltage, when overshot, reaches an upper limit value Vmax of a normal range (at time t2) as shown in (a) of FIG. 21. It is sometimes the case that the abnormality detecting unit 56 stops the operation of the power converting unit 3. In other words, the output voltage does not exceed the upper limit value Vmax during the overshooting if the forward voltages Vf have typical values (as indicated by a dot line in (a) of FIG. 21), but may sometimes exceed the upper limit value Vmax during the overshooting if the forward voltages Vf grow higher than the typical value (as indicated by a solid line in (b) of FIG. 21).
In an effort to prevent the abnormal stop of the power converting unit 3, the upper limit value Vmax of the output voltage may be set higher than the forward voltages Vf. In this case, the output voltage is increased rapidly when the loads 2 suffer from open failure (disconnection), and may sometimes exceed the upper limit value Vmax as shown in (a) of FIG. 22. The abnormal increase in the output voltage is likely to apply stresses to the circuit parts of the lighting device 1′, which may lead to failure of the lighting device 1′. If the abnormality detecting unit 56 is allowed to stop the operation of the power converting unit 3 in response to the instantaneous increase in the output voltage, there is a possibility that the abnormality detecting unit 56 may be erroneously operated due to a noise or other causes. Consequently, the output may become excessive because time required in determination gets longer. The failure of the lighting device 1′ can be prevented by using high-voltage circuit parts, however, this may lead to an increase in the size cost of the lighting device 1′.
Since the LEDs 21 and 22 show greater variations in the forward voltages Vf thereof, it is difficult to balance the upper limit value Vmax of a normal range of the output voltage and the forward voltages Vf so that the abnormal stop of the power converting unit 3 and the abnormal increase in the output voltage should not occur. In particular, the variations in the forward voltages Vf become greater in proportion to the number of serially-connected LEDs.
Further, due to the combination of poor bonding, faulty connectors with vibration or other causes, it is sometimes the case that the loads 2 are momentarily (several milliseconds) disconnected from the lighting device 1′ and then connected to the lighting device 1′ (hereinafter referred to as “load chattering”). In this case, the loads are turned off momentarily and the output voltage is increased as shown in (a) of FIG. 23. When the loads 2 are reconnected, the output voltage thus increased is applied to the loads 2, such that an excessive current may flow through the LEDs 21 and 22, possibly causing failure of the LEDs 21 and 22. At this time, the electric current flowing through the LEDs 21 and 22 becomes greater in proportion to the difference between the upper limit value Vmax of the output voltage and the forward voltages Vf. Therefore, it is desirable to set the upper limit value Vmax as small as possible while avoiding the abnormal stop of the power converting unit 3. This also makes it difficult to balance the upper limit value Vmax of a normal range of the output voltage and the forward voltages Vf.
The abnormal increase in the output voltage occurring due to the open failure of the loads 2 or the load chattering becomes problematic even in a lighting device 1′ having no load changeover function that does not employ the load changeover unit 55 and the bypass switch 4 shown in FIG. 18.